Too early to link Sandy to climate change

The world will need more cooperation in the coming years in weather forecasting, both to encourage developments in forecasting
capability and to gain the time needed to prepare for disaster

By J. Marshall Shepherd and John Knox

Illustration: Yusha

In the waning weeks of the North American hurricane season — a time when a superstorm is not expected to cause widespread damage to the eastern coast of the US — Hurricane Sandy is a grim reminder of the menace of extreme weather events. With the lowest central pressure of this year’s hurricane season, Sandy may have caused up to US$20 billion in damages, making it one of the costliest superstorms in history.

Sandy interacted with a weather system moving toward it from the east, posing difficult challenges for forecasters and nearly unprecedented weather conditions for the region. A similar storm hit New England 20 years ago. However, Sandy was worse, delivering hurricane-strength winds, drenching rains and severe coastal flooding throughout the populous mid-Atlantic and northeast corridor.

Some people will, of course, try to link Sandy with climate change. A similar rush to judgement occurred in the wake of massive tornado outbreaks in the US in recent years, even though the scientific literature does not offer strong support for such a connection. So, from the perspective of climate change, it is best to take a measured view of Sandy, lest hasty reaction harm scientific credibility.

However, that is little cause for comfort. According to the giant insurance company Munich Re, weather and climate disasters contributed to more than one-third of a trillion dollars in damage worldwide last year, and this year’s total may rival that amount. There is growing evidence of links between climate change and sea-level rise, heat waves, droughts and rainfall intensity, and, although scientific research on hurricanes and tornadoes is not as conclusive, that may be changing.

Indeed, recent reports by the UN Intergovernmental Panel on Climate Change and other scientific literature suggest that the intensity of tropical cyclones (that is, hurricanes) will increase as a result of warmer waters. And our atmosphere and oceans are, indeed, warming, with substantial residual heat stored in the ocean, to be released at some future time. A few studies have even suggested that tropical cyclones may be “wetter.”

It is quite certain that sea levels have risen over the last century, and continue to rise, in response to changing climate. And storm surges now ride on these elevated sea levels, amplifying flooding losses where they strike.

Sea surface temperatures along the US northeast coast are about 15°C above average, which helped to intensify Sandy just prior to landfall. At this point, it is premature to link the storm’s severity to warmer sea-surface temperatures, because regional variability is known to occur, but the link certainly is plausible.

Moreover, sea levels along the US northeast coast are rising up to four times faster than the global average, making the region more vulnerable to storm surges and flooding. And here the bottom line is that any coastal storm system will produce more flooding because of sea-level rise.

It should also be noted that an atmospheric weather pattern known as a “block,” a persistent area of high pressure that may have led to record melting in Greenland, was most likely the reason that Sandy moved inland rather than out to sea. It is too early to tell whether this blocking pattern is a manifestation of weather variability, a short-term climate variation, or the result of climate change.